In motor vehicles, radar systems are increasingly used in order to determine distances and relative speeds of objects in the vicinity of the motor vehicle. The named radar systems can be used to realize convenience functions (such as adaptive cruise control, or ACC) and safety functions (such as warning the driver in critical situations, or causing a full braking of the vehicle if an impact can no longer be avoided).
For these applications, the distance and relative speed of the objects, and the angle of the reflected radar waves, have to be determined precisely. In particular, the estimation of the angle, or direction of arrival (DOA), has to be carried out precisely for this purpose. The angle to be estimated can be an azimuth angle and/or an elevation angle. Compared to conventional single-input multiple-output (SIMO) radar systems, multiple-input multiple-output (MIMO) radar systems offer the advantage of a larger virtual aperture at the same or smaller geometrical size. Therefore, normally a greater precision of the estimated DOA can be achieved using an MIMO radar unit.
In general, however, the precision of DOA estimations in an MIMO radar unit decreases if the object is moving relative to the radar unit, because the movement produces an unknown phase shift (Doppler phase) of the base band signal, due to the Doppler effect.
Feger et al., “A 77-GHz FMCW MIMO Radar based on a SiGe Single-Chip Transceiver” (2009) discloses a MIMO radar unit in which receive and transmit antennas are positioned such that at least two antenna elements of a virtual array have the same geometrical position (redundant position). An unknown phase shift (Doppler phase) of the base band signal due to a target movement is estimated by calculating the phase difference of the antenna elements at the redundant positions. The estimated Doppler phase is subsequently used as a correction in an algorithm for the angle estimation.
However, in order to determine the unknown Doppler phase, only antenna elements at the redundant positions are used. Subsequently, all antenna elements are used to determine the angle at which the signal arrives. Therefore, a noise of the antenna elements at the redundant positions has a greater influence on the DOA estimation than does a noise at the rest of the antenna elements. As a function of the time multiplexing schema used, and of the configuration of the antennas, a DOA estimation of such a system can even be worse than with the use of a single transmit antenna.
Wintermantel, “Radar System with Elevation Measuring Capability” (2010) discloses a MIMO radar system having a plurality of transmit and receive antennas. The transmit antennas transmit numerous chirp sequences of short duration, for example 512 or 1024 chirps each with a duration of, for example, 10 μs. The transmitters transmit sequentially or simultaneously, but with different phase modulations. The distance, the relative speed, and the DOA are estimated using a three-dimensional discrete Fourier transform. In particular, in the case of a plurality of objects having the same distance and the same relative speed, the proposed system can carry out only imprecise angle estimations, and in addition cannot be used at all with a frequency-modulated continuous wave (FMCW) radar having long ramps. In addition, due to the chirp sequence design with rapid frequency changes, the hardware is relatively complex and expensive to realize.